With reference to FIGS. 1 and 2, a prior art ducted fan gas turbine engine is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an Intermediate Pressure (IP) compressor 13, a High Pressure (HP) compressor 14, combustion equipment 15, a High Pressure (HP) turbine 16, an Intermediate Pressure (IP) turbine 17, a Low Pressure (LP) turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the IP compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The IP compressor 13 compresses the air flow A directed into it before delivering that air to the HP compressor 14 where further compression takes place.
The compressed air exhausted from the HP compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the HP, IP and LP turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The HP, IP and LP turbines respectively drive the HP and IP compressors 14, 13 and the fan 12 by suitable interconnecting rotors.
The outermost rotor is the High Pressure (HP) rotor 20 also known as the HP compressor drive cone. This rotor 20 connects the HP compressor 14 with the HP turbine 16. The HP rotor 20 is relatively large in diameter and short in length making it very stiff. Due to this stiffness, the HP rotor 20 can be supported on two bearings—a HP thrust ball bearing 24 at the front (upstream) carrying the thrust and an HP radial (roller) bearing 25 at the rear (downstream).
The next innermost rotor is the Intermediate Pressure (IP) rotor 26 which connects the IP compressor 13 to the IP turbine 17. This rotor 26 has a smaller diameter and is longer than the HP rotor 20 making it too flexible to only be supported at its ends. Therefore, three bearings are provided to support the IP shaft 26: an upstream IP radial bearing 27 in front of (upstream from) the IP compressor 13, an IP thrust bearing 28 aft of (downstream from) the IP compressor 13 and a downstream IP radial bearing 29 near the IP turbine 17.
Finally, the low pressure (LP) rotor 30 is innermost, connecting the LP turbine 18 to the fan 12. The LP rotor 30 is even longer and has an even smaller diameter than the IP rotor 26 and also requires support at three locations; a upstream LP radial bearing 31 aft of (downstream from) the fan 12, an LP thrust bearing 32 aft of (downstream from) the IP compressor 13 and a downstream LP radial bearing 33 aft of (downstream from) the LP turbine 18.
Thrust loads arise in a gas turbine engine as the result of pressure imbalances. For example, a compressor has a higher downstream pressure than upstream pressure which forces the compressor upstream (towards the intake) whereas a turbine has a higher upstream pressure than downstream pressure which forces the turbine downstream (towards the exhaust nozzle).
The LP, IP and HP rotors 20, 26, 30 are subjected to axial forces arising from the downstream forces generated by the turbines 16, 17, 18 and the upstream forces generated by the compressors 14, 13 or fan 12. Typically, a balance piston 34 is used to reduce the thrust load to a value that avoids overloading of the thrust bearings. However, the thrust loads remain high and uncertain.
The thrust loads are often at their maximum during the periods of highest power output for the engine. In a gas turbine engine providing jet propulsion for an aircraft, this period of maximized power output can occur when the aircraft is taking-off and/or climbing to a cruising altitude. The thrust loads can change direction (passing through a zero load point) during a flight cycle.
The thrust bearings can be positioned to support the rotors 20, 26, 30 against these thrust loads. A thrust bearing typically comprises an inner and outer race, a cage and a set of roller elements, the roller elements being spheres (or balls) which are contained within a raceway formed in one or both of the races with the cage maintaining the spacing between the balls.
It is known to use inter-rotor thrust bearings with one race connected to rotor and one race connected to another rotor to transfer loads between rotors. For example, the Rolls-Royce Trent 900, and Trent 1000 engines use a thrust bearing mounted between the IP and LP rotors to transfer axial forces on the LP rotor through the IP rotor to a thrust bearing mounted on the IP rotor.
There is a desire for an improved bearing load sharing system for the management of axial thrust forces in bearings on rotors within a gas turbine engine.